1. Field of the Invention
The present invention relates to a multilayer crosshead for a blow molding machine and more particularly to one in which a membrane discontinuity in an annular multilayer passage is monitored using a plurality of pressure sensors, layer thickness is maintained constant by equipping an inflow inlet of the annular passage with a throttle valve, parison temperature is lowered by equipping a crosshead body with a cooling jacket with a built-in heater so that a blow molding cycle is reduced, and adhesive strength of a adhesive layer between a main material layer and a barrier layer is increased by elongating the length of a path of the layers after being merged into a multilayer.
2. Description of the Related Art
As a conventional crosshead for a blow molding machine, for example, a structure disclosed in Japanese Patent Laid-open No. 4-316808 is known, in which a main material layer, a barrier layer, an adhesive layer, etc., are extruded into each annular passage of a crosshead from each of independent extruders so that the layers are merged into a multilayer to be discharged from a die slit as a parison via a die discharge-outlet.
A conventional multilayer crosshead for a blow molding machine configured as above has the following problems. That is, since only one pressure sensor for a main material, etc., supplied to the annular passage is disposed at an accumulator of the extruder or at an inlet of the multilayer forming section, there is a problem that the pressure difference between an inflow inlet of the annular passage and a weld portion opposing the inflow inlet cannot be completely sensed. Consequently, when pressure in the weld portion is low, membrane discontinuities may occur in the weld portion, and that for example, when a parison for blow molding a container such as a gasoline tank is formed, there is no production engineering method for sensing the membrane discontinuities.
Since a valve for adjusting a flow rate is not provided in an inflow inlet of the annular passage, there is a problem that variations of thickness are produced among each layers, thus a designed parison strength has not been obtained.
Since the crosshead is not provided with a cooling means, a parison in a high temperature state is supplied to a metallic mold, there is a problem that a long time is required for cooling the molding in the metallic mold during blow molding, thus a molding cycle has not been shortened.
Further, since a distance between the merging position of the materials in the crosshead flowing down-from the annular passage and the die discharge-outlet is not especially considered in accordance with an adhesive property of the multilayer parison, when a material velocity in the die slit is increased in accordance with reduction of a molding cycle, sufficient time and pressure necessary for adhering the barrier layer and the adhesive layer are not obtained, consequently, there is a problem that adhesive strength between layers of a molded parison becomes insufficient.
Accordingly, it is an object of the present invention to solve the above-described problems and particularly to provide a multilayer crosshead in which a membrane discontinuity in a multilayer annular passage is avoided using a plurality of pressure sensors, layer thickness is maintained constant by equipping an inflow inlet of the annular passage with a throttle valve, a parison temperature is lowered by equipping a crosshead body with a cooling jacket so that a blow molding cycle is shortened, and adhesive strength among a main material layer, a barrier layer and an adhesive layer is increased by elongation of the length of a path of the die.
In accordance with the present invention, a multilayer crosshead comprises a plurality of extruders; a crosshead body; a plurality of annular passages formed in the crosshead body; a die disposed in a bottom portion of the crosshead body including a die slit having a die discharge outlet and being communicated with each of the annular passages; wherein plural materials being different from each other extruded by the plurality of extruders are supplied to the plurality of annular passages so as to flow down from the die slit to form a parison, the multilayer crosshead further comprising pressure sensors disposed in an inflow inlet of the annular passage and in a weld portion of the annular passage opposing the inflow inlet so that the pressure difference between the pressure sensors is sensed. Each of the pressure sensors may include a pressure sensing portion disposed in a concave portion adjacent to the annular passages, and a signal converter disposed outside of the crosshead body being connected to the pressure sensing portion via the length of lead wire. A multilayer crosshead may further comprise a throttle valve disposed in the inflow inlet of each of the annular passages for adjusting a flow rate of the material; and a block having the annular passages, wherein the throttle valve and the block are detachably disposed in the crosshead body. The crosshead body may be provided with a cooling jacket with a built-in heater. The cooling jackets with built-in heaters are disposed inside and outside of the crosshead body, outside of the die, and inside of the core.
At least main material layers, a barrier layer, a pulverized layer, and adhesive layers may be used as the materials, wherein a length from a position, in which these layers are merged, to the die discharge outlet of the die slit, may be established so as to unitarily adhere these layers with each other.